Method of combusting a residual fuel utilizing a two-stage air injection technique and an intermediate steam injection step



1967 G. R. BOHENSKY ETAL 3,359,723

METHOD OF COMBUSTING A RESIDUAL FUEL UTILIZING A TWO-STAGE AIR INJECTION TECHNIQUE AND AN INTERMEDIATE STEAM INJECTION STEP Filed Oct. 29, 1965 C'OMBUSTOR 2o FUEL AIR

COMPRESSOR EXHAUST WASTE HEAT BOILER AUXILIARY FUEL '4 amaa $22 322" WESLEY o.' NILES CHARLES w. SIEGMUND wpjm PATENT ATTORNEY United States. Patent METHOD OF COMBUSTING A RESIDUAL FUEL UTILIZING A 'TWO-STAGE AIR INJECTION TECHNIQUE AND AN INTERMEDIATE STEAM INJECTION STEP George R. Bohensky, Parsippany, Walter A. Herbst, Union, Wesley D. Niles, Roselle Park, and Charles W. Siegmund, Morris Plains, N.J., assignors to Esso Research and Engineering Company, a cor oration of Delaware Filed Oct. 29, 1965, Ser. No. 505,606 5 Claims. (Cl. 60-39.05)

ABSTRACT OF THE DISCLOSURE The present invention is concerned with a technique and apparatus for burning a high sulfur, high ash residual fuel which comprises adding about a stoichiometric amount of steam in an initial zone, thereafter adding steam passed about said initial zone to the combustion product in a secondary zone and thereafter adding addi tional air to said combustion products in a tertiary zone.

The present invention is broadly concerned with an improved gas turbine engine and with its method of operation. The invention is more particularly concerned with a gas turbine engine designed for utilizing a liquid petroleum fuel which is characterized by being a residual fuel containing a relatively high concentration of sulfur and ash forming constituents. In accordance with the present invention, a gas turbine engine is designed so as to be operated using a relatively low quantity of excess air upstream in the combustor and in combination with the addition of additional air downstream in the combustor, and also in combination with the use of steam. In accordance with one specific adaptation of the present invention, a controlled amount of steam injection is used to cool the combustor chamber walls and also the combustion products in the first twozones of a combustor while additional air is utilized in the final zone of the combustor. Thus, in accordance with the present invention, a gas turbine engine is operated by burning an ashcontaining fuel with essentially the stoichiometrically correct amount of air (0 to 3% excess) followed by quenching of the hot gases by steam or a mixture of steam and air added downstream from the primary combustion zone.

In the operation of a conventional, open cycle, gas turbine engine, it is necessary to utilize relatively large quantities of excess air with the combustion gases in order to keep the temperatures within the temperature limits which materials can withstand. For example, stoichiometric combustion of a typical fuel results in a temperature in the range of about 3400 F. to 4000 F. while the present limitation of the turbine blade materials in industrial gas turbine engines is in the range of about 1400 F. to 1600 F. For this reason, in conventional turbine engines as much as 500% excess air is used in order to lower the, temperatures to within a range which the materials can withstand. This results in inefiiciencies and other operating problems.

Also, for economic reasons it is desirable to utilize inexpensive petroleum fuels in turbine engines, particularly in stationary power plants. Such fuels include petroleum liquid fractions boiling above about 800 F. and having viscosities above about 400 SS at 150 F. These fuels contain a substantial amount of ash-forming components which include compounds of sodium, calcium, nickel, iron and vanadium. These fuels, in most instances, also have sulfur concentrations in the range from about 0.5

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to 5% by weight. Thus, these fuels when used in gas turbine engines tend to be excessively corrosive and highly ash forming.

It is known that if near stoichiometric amounts of air, 0 to 3% excess air, are used in the combustion of residual fuels of this particular type containing vanadium and sodium salts, the corrosion and fouling characteristics associated with these fuels are substantially decreased. Therefore, for this reason it is much preferred to use a minium amount of excess air.

Such fuels, for example, when burned in the presence of large amounts of air (e.g. 5 to 500% above stoichiometric requirements) produced ash and products of combustion that form deposits and corrode the downstream hot parts of the engine (e.g. transition ducts, fixed nozzles, turbine blades). On the other hand, when the amount of air is limited to 0 to 3 wt. percent excess over the theoretically correct amount, the deposit-forming and corrosive compounds are not formed. These effects are illustrated by the following data obtained with a petroleum residual fuel containing 0.1 wt. percent ash.

EFFECT OF COMBUSTION AIR ON DEPOSIT AND COR- ROSION CHARACTERISTICS OF THE ASH FROM A RESIDUAL TYPE PETROLEUM FUEL [Fuel 0.1 wt. percent ash (includes 0.035 wt. percent V,

0.007 wt. percent Na)] Amount of Combust. Air Ash Deposits, Corrosion Wt. Loss,

Percent Above Theoret. rug/em. mgJcnr-Ka) Reqmns.

(* Stainless steel 347 (18% chromium, 8% nickel).

However, stoichiometric combustion of a typical fuel results in a temperature in the range of about 3600 F. This is much too high for current turbine blade materials, which cannot practically handle gases hotter than about 1200 to 1800 F. Thus, means of cooling these combustion products to 1200 to 1800" F. must be employed. In current turbine engine design this is done by adding large amounts of excess air (up to 500% over stoichiometric requirements) to the combustor. This results in inefficiencies (because of the work required to compress this large amount of air) and gives the aforementioned harmful ash and combustion products.

In the method of operation disclosed herein this cooling is accomplished by steam an-d/ or steam and air added downstream from the primary combustion zone. The steam is produced in a waste heat boiler heated by the turbine exhaust and can be generated at pressures above the turbine cycle pressure. It can, therefore, be added directly to the cycle for cooling purposes and adds to the massv of the working fluid without requiring the compression work characteristic of air cooling. This results in increased power and thermal efliciency.

One form of a conventional gas turbine combustor comprises an exterior or outer shell and a substantially concentrically disposed inner shell or casing. The combustor has generally three main zones. Fuel is injected in the primary zone along with air where combustion occurs and where the temperature exceeds about .3000 F. Excess air is passed about the exterior of the casing walls of the primary zone to cool the walls of this primary zone. In the secondary zone of a conventional casing, the casing contains louvers, or slots, through which some of the excess air is admitted to within the casing which provides turbulence for complete combustion. The tertiary zone is essentially a quench zone where the major portion of the excess air is introduced to cool the combustion gases and provide the proper temperature for the fluid for utilization in the gas turbine cycle.

In accordance with the present invention, steam is utilized to cool the casing walls in the primary and secondary zones and thus permit the combustion to be carried out with about 3% excess air. In the tertiary zone where the combustion is essentially complete, in accordance with the present invention, the hot gases mixed with steam, are quenched by additional steam and/ or air which has bypassed the first two zones of the combustor basket or casing. One way of operating in this manner is by the use of a combustor comprising an outer shell, an inner shell or casing or combustor basket, and a third intermediate substantially concentrically disposed shell, the length of which is less than the length of the outer shell or casing. I

The unique combustor and method of operation will be readily understood by reference to the drawing illustrating one embodiment of the same. Referring specifically to the drawing, air is introduced into the compressor 10 by means of line 1. In accordance with the present invention, a stoichiometric portion of the compressed air at a pressure in the range from about 75 to 200 p.s.i., such as about 90 psi, is introduced into combustor 20 by means of line 2. Combustor 20 comprises an outer shell 30, an inner shell or casing 40, and an intermediate shell 50. Thus, an annular area 31 is provided between shells 30 and 50, and an annular area 32 between shells 40 and 50. The stoichiometric portion of the air is introduced into primary zone 1 within casing 40 of combustor 20. Fuel is also introduced into primary zone I of casing 40 by means of line 3. Conventional means are utilized to secure the combustion of the fuel within casing 40. As mentioned, the amount of air introduced into primary zone I is below excess air, preferably in the range of from stoichiometric to 3% excess air.

In accordance with the present invention, steam, preferably produced as hereinafter described, is introduced into an annular area 32 about primary zone I by means of line 4. This steam flows through annular area 32 and serves to cool the walls of primary zone I. Substantially complete combustion occurs in primary zone I within casing 40. The hot combustion gases flow into secondary zone II of casing 4-0 wherein the hot gases are mixed with the steam which flows into secondary zone II through slots or louvers 5, which provide communication from area 32 to within casing 40.

In accordance with the present invention, a portion of the compressed air from compressor is introduced into annular area 31 of combustor 20 which is positioned about annular area 32. This compressed air flows through annular area 31 and is introduced by means of apertures or slots 6 into tertiary zone III of casing 40 wherein the same is mixed with the combustion gases and the steam. This added air serves as a quench for the steam and the combustion products. The combustor products are withdrawn from combustion zone 20 by means of line 7 and function to drive turbine 80 in a conventional manner. Turbine 80 is linked by suitable means 8 in order to drive compressor 10. The load 60 is driven by suitable means 9 from turbine S0.

The spent turbine gases are withdrawn from turbine 80 by means of line 11 and passed into waste heat boiler 70 wherein they are utilized to generate steam which is withdrawn by means of line 4 and passed into annular area 32. Suflicient water is introduced into waste heat boiler 70 by means of line 12 while spent exhaust gases are withdrawn from waste heat boiler 70 by means of line 13.

The advantage of the present invention is that it is possible to burn residual fuel without the attendant corrosion problems presently limiting turbine applications with this type of fuel. As residual fuel is considerably less expensive, the technique permitting its use represents a sizeable savings in economics. An added advantage of this cycle is the increase in efficiency obtained by injecting steam and increasing output work by increasing mass flow without increasing the compressor load.

As pointed out heretofore, the amount of air mixed with the fuel in primary zone I is substantially stoichiornetric and preferably should not exceed about 3% excess air.

The amount of steam introduced into secondary zone II is in the range from about 3 to 20 pounds of steam per pound of fuel, preferably about 6 to 10 pounds of steam per pound of fuel to the primary combustion zone. The pressure of this steam is in the range of 75 to 200 p.s.i. The amount of air introduced into tertiary zone III of casing 40 is from 0 to 50 lbs. per lb. of fuel to the combustor preferably 2535 lbs. per lb. of fuel. Under these conditions the temperature of the gases withdrawn from casing 40 and utilized in turbine 50 is in the range from about 1350 to 1700 F. and the pressure is in the range from 75 to 200 p.s.i. Under some conditions the recoverable heat in the exhaust gases may not be sufficient to provide all the steam required in this process. Additional steam may be generated in an oil-fired boiler which may be separate from, or a part of, the waste heat boiler 70. This auxiliary fuel will be supplied through line 14 at a rate equivalent to 0 to 1 lb. of auxiliary fuel per pound of primary fuel burned in the turbine combustor. Also as pointed out heretofore, the present turbine engine and technque is particularly adapted for the combusting of heavy residual fuels which contain in the range from about 50 to 1500 as, for example, 1000 parts per million of ash and wherein the vanadium content is greater than 2 as, for example, in the range from about 60 to 1000 parts per million of fuel.

Thus the basic concept of the present invention is to burn a fuel containing harmful trace components (i.e. metals and sulfur) with stoichiometric amouns of air. This keeps the combustion products of the metals and sulfur in a low valence state where they do not form the harmful deposits characteristic of the higher valence states obtained with large amounts of excess air. The products from the stoichiometric combustion, however, are too hot to be used directly in a turbine and therefore must be cooled. If air is used as a quench at high temperatures, the ash and sulfur will be oxidized and the benefits of the stoichiometric combustion will be lost. This is avoided by quenching first with steam and then adding air if necessary. The amount of steam required is that necessary to reduce the temperature of the combustion products to a point where they will not oxidize in the presence of added air.

In order to further illustrate the present invention Attachr nent I shows the relative amounts of steam and air required for 1500 F. turbine inlet temperature under one set of turbine conditions. The various cases are as follows:

Case 1.Conventional turbine operation where cooling is done by the use of excess air over that required for combustion. The combustion air and coolant air are supplied by the compressor.

Case 2.-Steam from waste heat boiler replaces part of the air for cooling. The amount of steam is held at 10% of mass cycle flow. The additional cooling required to reach 1500 F. turbine inlet temperature is accomplished by air from the turbine compressor.

Case 3.Steam from waste heat boiler is added to cycle for cooling. Total air flow held same as in Case 1. Fuel flow is increased as allowed by increased cooling from added steam.

Case 4.-The maximum amount of steam that can be produced from the waste exhaust heat is added to cycle. Fuel flow is same as Case 1. Air flow decreased since steam provides part of cooling.

Case 5.Maximum amount of waste heat steam added to cycle. The same as Case 4, except total air flow held c stant s. same as Case 1 and fuel flow increased.

Case 6.-flteam alone is used for cooling. Fuel flow same as Case 1. Coolant air eliminated. This case requires more steam than is available from Waste heat in exhaust i.e. supplemental fuel oil must be fired under steam boiler.

Case 7.--Liquid water and maximum waste heat steam used for cooling. Coolant air eliminated. Fuel flow same as Case 1.

The desirable cases are 2, 3, 4 and 5 in which the amounts of steam used are limited to that produceable from the waste heat in the exhaust gases. While it would be difficult to justify the production of additional steam specifically for this purpose, under certain conditions waste steam may be available from outside sources which is illustrated in Case 6.

zone, and thereafter driving a turbine with the gaseous mixture.

2. Process as defined by claim 1 wherein said air added to said tertiary zone is passed about the steam added to said secondary zone.

3. Process as defined by claim 1 wherein the amount of steam added to said secondary zone is in the range of 6.0 to 10 pounds of steam per pound of fuel introduced into said primary zone.

4. Process as defined by claim 1 wherein a stoichiometric amount of air is introduced into said primary zone.

5. Process as 'denfied by claim 1 wherein additional steam is supplied to said secondary zone by burning supplemental fuel oil in a waste heat boiler at a quantity to 0.7 pound of supplemental fuel per pound of fuel to the combustor.

ATTACHMENT I.TURBINE OPERATION WITH ADDED STEAM lBases: Turbine press. ratio 9.7; turbine inlet temp. 1,500 F.; Compression discharge temp. 650 F.; turbine exhaust temp. 1,000

F.; waste heat boiler exhaust 400 F.; adiabatic flame temp. 4,000 FJ Case 1 2 3 4 5 6 7 All Air Air+10% Air+10% Air-l-Max. Air+Max. All Water+Max. Conven- Steam Steam Waste Heat Waste Heat Steam Waste Heat tional Steam Steam Steam Fuel Flow, Lbs 1 1 1. 7 1 2.0 1 1 Combustion Air, Lbs 14. 7 14. 7 24. 4 14. 7 29. 4 14. 7 14. 7 Cooling Air, Lbs 67 37 58 25 53 0 0 Total Air, Lbs 82 52 82 40 82 15 Steam, Lbs.:

From waste heat boiler 0 6 11 10 21 13 5 From fuel fired boiler 0 0 0 0 0 8 0 Auxiliary Fuel Oil Required, Lbs 0 0 0 0 0 0,5 0 Liquid Water 0 0 0 0 0 0 5 To combustor.

What is claimed is:

1. Improved process for the operation of a gas turbine engine which comprises introducing a residual fuel boiling above about 50 F. and having an ash content to 1500 parts per million of fuel, a vanadium content of 2 to 400 parts per million of fuel, and a sulfur concentration in the range from about 0.5 to 5% by weight, and less than about 5% excess air into a primary zone of a combustor and combusting the same to produce combustion products, passing about 3 to pounds of steam per pound of fuel about said primary zone and mixing said combustion products with said steam in a secondary zone, then adding about to pounds of additional air per pound of fuel to said combustion products and said steam in a tertiary References Cited UNITED STATES PATENTS CARLTON R. CROYLE, Primary Examiner. 

1. IMPROVED PROCESS FOR THE OPERATION OF A GAS TURBINE ENGINE WHICH COMPRISES INTRODUCING A RESIDUAL FUEL BOILING AWBOVE ABOUT 50* F. AND HAVING AN ASH CONTENT TO 1500 PARTS PER MILLION OF FUEL, A VANADIUM CONTENT OF 2 TO 400 PARTS PAER MILLION OF FUEL, AND A SULFUR CONCENTRATION IN THE RANGE FROM ABOUT 0.5 TO 5% BY WEIGHT, AND LESS THAN ABOUT 5% EXCESS AIR INTO A PRIMARY ZONE OF A COMBUSTOR AND COMBUSTING THE SAME TO PRODUCE COMBUSTION PRODUCTS PASSING ABOUT 3 TO 20 POUNDS OF STEAM PER POUND OF FUEL ABOUT SAID PRIMARY ZONE AND MIXING SAID COMBUSTION PRODFIF-01 UCTS WITH SAID STEAM IN A SECONDARY ZONE, THEN ADDING ABOUT 25 TO 35 POUNDS OF ADDITIONAL AIR PER POUND OF FUEL TO SAID COMBUSTION PRODUCTS AND SAID STEAM IN A TERTIARY ZONE, AND THEREAFTER DRIVING A TURBINE WITH THE GASEOUS MIXTURE. 